Fair Value Measurements and Investments - Investment Income (Detail) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Net Investment Income [Line Items]
Interest and dividend income on cash and investments	$ 80	$ 247	$ 173
Realized gains (losses), net on debt securities
Realized gains (losses), net	4	1	0
Equity securities:
Unrealized gains (losses), net	115	117	2
Investment income	200	383	470
Donation
Equity securities:
Realized gains (losses), net	0	0	193
Excluding Donation
Equity securities:
Realized gains (losses), net	$ 1	$ 18	$ 102

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